1. Field of the Invention
The present invention relates in general to a display optical system, more particularly, to an illuminating apparatus in a projection-type display optical system based on a DMD.
2. Discussion of the Background Art
As image projection apparatuses also called projectors are widely used in many fields, diverse types of projectors are currently under development or already came into the market. A recent trend in the technologies for image projection apparatuses is to improve brightness and to develop small size/light weight image projection apparatuses.
An optical system of the image projection apparatus includes a lamp being used as a light source, an illumination unit for illuminating a light source from the lamp to an image display device, and a projection unit for enlarging and for projecting images displayed on the image display device onto a screen.
A high-pressure mercury lamp is usually used for the lamp. As for the image display device, liquid crystal display device or DMD (Digital Micromirror Device) is widely used.
The above-cited DMD, having a two-dimensional array of a number of pixels each having a micromirror, controls the tilt of each mirror individually through the effect of electrostatic field caused by a memory element arranged respectively for each pixel and varies the angle of reflection of reflected light ray thereby causing on/off state.
Depending on the number of image display devices used in the projection type display, the optical system is divided into single panel-, 2 panel-, and 3 panel-optical systems. Keeping abreast with the recent trend in small size/light weight and low-price devices, 1-chip image display devices are now used.
There are three methods for the construction of an image projection apparatus with the 1-chip image display device.
First, the display device can include red, green, and blue (R, G, B) color filters. Second, a light can be divided into R, G, and B colors in outside and at the same time, illuminated on a display device. Third, a light can be divided into R, G, and B colors and illuminated at regular intervals.
Out of the above-described methods, the present invention will be based on the third method, i.e. the light is divided into R, G, and B colors and illuminated at regular intervals.
With the application of the third method, response speed of the 1-panel image display device needs to be at least three times faster than that of the 3-panel image display device. Among the current image display devices DMD™ will satisfy this condition.
FIG. 1 illustrates a simplified structure of a related art DMD, and optical operation states of the DMD as a display device.
As shown in FIG. 1, DMD 10 is composed of micromirrors 12 (each micromirror represents one pixel), and each of the micromirrors 12 is in ±Θ tilt mode according to an electric signal. The currently commercialized tilt angle of the micromirrors is 10 or 12 degrees.
Although, in reality, the micromirrors 12 tilt in a diagonal axis of square pixels, for convenience of description, an assumption is made that the tilt of the micromirrors 12 is operated with respect to a vertical axis.
Typically, when light rays reflected off the micromirrors 12 are directed to a projection lens 30 and form a magnified image on a screen, the surface of the DMD 10 and the optical axis of the projection lens 30 should be positioned in the vertical direction. In general, in the horizontal direction of the DMD 10 the center of the DMD 10 and the optical axis of the projection lens 30 coincide with each other. In the vertical direction of the DMD 10, on the other hand, an upward projection is applied for the sake of convenience to decenter optical centers. However, in the related art DMD shown in FIG. 1, it is assumed that the optical centers are not decentered.
Referring to FIG. 1, for the micromirrors of the DMD to be an optically on state (white) under the above condition, a chief ray of illuminating rays should incident on the surface of DMD 10 in such a manner that the chief ray can be emitted perpendicular to the surface of DMD 10 especially when the tile angle of the micromirrors 12 of the DMD 10 in the on state is ±Θ. In this case, the incidence angle of the illuminating ray on the DMD surface should be 2Θ.
Under the above-described structural conditions for the DMD type projection optical system, light rays in the off state are emitted at a 4Θ tilt angle with respect to the optical axis of the projection lens 30. Thus, the light rays cannot transmit the projection lens 30, and thus cannot project light on the screen, resulting in a black screen.
FIG. 2a is a plane view of one embodiment of a related art projection optical system based on a single-chip DMD, and FIG. 2b is a plane view and a side view of a color wheel in a general color filter in a time-sharing system.
As depicted in FIG. 2a, as for a light source a lamp 80 having an ellipsoidal reflective mirror 82 attached thereto is used, and light rays from the light source are focused on an incident surface of a rod lens 60.
Arranged between the lamp 80 and the rod lens 60 is a color wheel 70 for separating the light into R, G, and B colors in sequence.
The color wheel 70, as shown in FIG. 2b, is attached to a rotatory motor 72 like a disk, and sequentially filters R, G, and B colors of light rays as the motor rotates.
Because an area with a least color filtering is where the light rays from the lamp 80 are focused on the incident surface of the rod lens 60, the color wheel 70 is positioned before the incident surface of the rod lens 60.
Therefore, when a light ray having been filtered to a specific color through the color wheel 70 incidents on the rod lens 60, the light ray goes through several times of reflection inside of the rod lens 60, and transmits the rod lens 60. Then, the transmitted light ray is scattered over the entire emitting surface.
In other words, the light ray from the light source is progressed or decentered to the emitting surface of the rod lens 60, and as a result thereof, the emitting surface becomes a surface light source having a secondary uniform contrast distribution.
The emitted light from the rod lens 60 is transmitted through a first and second illuminating lens groups 50 and 40 and a TIR (Total Internal Reflection) prism 20, and forms a proper-size image of the emitting surface of the rod lens 60 on the image display device, namely the DMD surface. In this manner, the DMD surface obtains uniform contrast distribution.
Referring back to FIG. 2a, the TIR prism 20 is formed by setting two prisms apart with a slight air gap in between. Thus, an incident light is totally reflected off the first prism surface, and incidents on the DMD 10. The DMD 10 then emits the incident light at a different emission angle from the incident light by the tilt pixel micromirrors in on state (white), whereby the light does not experience total internal reflection but is transmitted to the outside again.
Thusly emitted light transmits the projection lens 30 and forms a magnified image on the screen.
In consideration with the total internal reflection from the first boundary surface of the illuminating ray and the operational characteristics of the TIR prism 20 for transmitting a white ray from the DMD 10 through the secondary boundary surface, it becomes important to maintain the telecentric characteristic of the illuminating ray.
However, the related art projection type optical system illustrated in FIG. 2a has several shortcomings. For instance, variable reflectivity and transmittance in dependence of the beam angle of the illuminating ray deteriorates light transmission efficiency, and an increased diameter of the projection lens 30 due to telecentric characteristic of the illuminating ray consequently increases cost of manufacture. Besides, the micromirrors of the DMD 10 are put in zero state, noises are generated by diffraction, and contrast is also degraded as light transmission is increased.
FIG. 3 illustrates another embodiment of a related art projection type optical system using a single-chip DMD.
Particularly, FIG. 3 illustrates an image projection apparatus without the TIR prism 20, to overcome the shortcomings found in the projection type optical system of FIG. 2.
As for the image projection apparatus without the TIR prism 20, the secondary illuminating lens group 40 can be utilized either in a glass type or in a mirror type. Since optical principles are basically same, it will be more necessary to discuss the structure of a reflective mirror lens.
Same operational principles of the projection optical system shown in FIG. 2 are also applied to the projection optical system of FIG. 3, more specifically, until the rod-shape tube rod lens 60 out of the system. Also, the illuminating lens 80 ensures that a chief ray of the illuminating rays emitted from the rod lens 60 incidents at an angle of 2Θ upon the DMD surface.
However, the projection optical system of FIG. 3 differs from the projection optical system of FIG. 2 in that a total reflection mirror 90 for changing a light path is installed in between the first illuminating lens group 50 and the second illuminating lens group 40. As a result, the light path of the first illuminating lens group 50 and the light path between the second illuminating lens group 40 and the DMD 10 are overlapped, and the entire optical system becomes more compact.
In addition, the optical system shown in FIG. 3 is no longer subject to telecentric limitation of illuminating rays by not including TIR prim 20. Accordingly, when incidenting on the surface of the DMD 10, chief rays at each objective space on the emitting surface of the rod lens 60 do not have to maintain the telecentric relation with other rays, but can be converged on the DMD surface.
In a practical sense, the converging illumination design is necessary to reduce the size of the incident surface of the projection lens 30 so that optical interference is not caused by the overlapped projection lens 30 and the mirror type lens (the second illuminating lens group) 40.
The optical system without the TIR prism 20, compared to the optical system with the TIR prism 20, is smaller, less costly, and has an improved contrast and brightness uniformity.
When the mirror type lens is used as the second illuminating lens group 40, however, the rod lens 60, the optical axis of the first illuminating lens group 50, and the optical axis of the second mirror type lens 40 may be coincident. In that situation, a reflected ray from the mirror type lens 40 travels back to the optical axis direction of the first illuminating lens group 50, which consequently causes the optical interference.
To obviate the above described problem, another embodiment of a related art projection type optical system shown in FIG. 4a introduces an idea of twisting the optical direction of a reflected light at Ψ degree angles, by rotating the mirror type lens 40 in Ψ/2 degrees with respect to an intersection between the mirror type lens 40 and the optical axis of the rod lens 60.
Here, the illuminating image-based surface on the emitting surface of the rod lens 60 is actually tilted at a certain degree angles from the DMD surface. Therefore, the illuminating image on the DMD surface 10 takes on a distracting keystone shape, as shown in FIG. 4.
Keystone distortion is caused when the illumination area and the actually effective DMD surface are not at one angle (i.e. The projected image looks like a trapezoid although it should be a rectangle). In this case, a loss in light rays is inevitable.
Also, keystone distortion problems differentiate illuminance according to the DMD 10 positions, and this resultantly deteriorates brightness uniformity on the screen.
This keystone distortion also exists in the optical system with the TIR prism 20 shown in FIG. 2a because the first illuminating lens axis is not perpendicular to the DMD surface axis and because an illuminating ray has an incidence angle of 2Θ on the DMD surface.